Various retarder films are used in liquid crystal display devices for performance increase. A retarder film is installed on the liquid crystal display so that a retardation axis is inclined by specific various angles with respect to a polarized light transmission axis of a polarizer and a polarized light transmission axis of a liquid crystal cell in order to fully exert its functions. The inclination angle of the retardation axis is an angle not in parallel with nor perpendicular to a side of the display.
As a method of obtaining a retarder film oriented not in parallel with nor perpendicular to the side, a method in which a lengthy stretched film which is obtained by orienting a transparent resin film by longitudinal stretching or lateral stretching is obliquely cut in a rectangular shape with a predetermined angle to the side of the stretched film is widely known. However, with this method, even if the maximum area is to be obtained, a lot of cutting loss is generated, and use efficiency of the stretched film is low.
On the other hand, a lengthy stretched film obliquely oriented with a predetermined angle can be cut in parallel with the side, and the use efficiency of the stretched film is increased. Various methods of obtaining such film obliquely oriented to the side by stretching have been proposed.
For example, Patent Document 1 discloses, in a stretching method of an optical polymer film in which both ends of a continuously supplied polymer film are held by a holding means and a tension is applied to stretch the film, while the holding means is advanced in the longitudinal direction, the stretching method of an optical polymer film characterized in that stretching is carried out while a relation of |L2−L1|>0.4W is satisfied among a locus L1 of the holding means from a substantial holding start point to a substantial holding release point on one end of the polymer film, a locus L2 of the holding means from a substantial holding start point to a substantial holding release point on the other end of the polymer film and a distance W between the two substantial holding release points, bearing properties performance of the polymer film is maintained and in presence of the state of a volatile content of 5% or more, and then, the volatile content is lowered while the film is made to shrink.
Patent Document 1: Japanese Patent Laid-Open No. 2002-86554 (U.S. Pat. No. 6,746,633)
Patent Document 2 discloses a manufacturing method of a lengthy optical film obtained by stretching a lengthy film made of a thermoplastic resin and having an orientation axis being in a direction not in parallel with nor perpendicular to a take-up direction of the lengthy film, characterized in that in a region where the film is substantially stretched, traveling speeds are equal and traveling distances are different at opposing both ends in the width direction of the film, and stretching is carried out so that at least one of a pair of jigs holding the both ends in the width direction of the film moves on a rail in a corrugated shape with respect to a film surface. Moreover, Patent Document 2 describes that the stretching step may be repeated several times or the stretching step may be carried out after stretching in the longitudinal direction or lateral direction in advance.
Patent Document 2: Japanese Patent Laid-Open No. 2003-232928
However, with these obliquely stretching methods, wrinkles or twists may occur obliquely and thickness tends to be uneven easily. Thus, it has been substantially impossible to obtain a wide film with a uniform thickness in the width direction having a width of 1300 mm or more and uniform orientation without fluctuation by an inclined orientation angle of particularly 40° or more to the take-up direction. Therefore, it has been impossible to industrially mass-produce a lengthy and wide optical film with an oblique orientation axis (direction largely offset from the width direction or longitudinal direction of the film).